Lights of the type described above are known in which the electronic control device substantially comprises a driving stage, which is connected to a lighting branch and is configured so as to adjust the drive current through the LEDs present in the lighting branch itself on the basis of a control signal, and a current adjustment stage, which is configured so as to supply the drive current to the lighting branch and to generate the control signal at the same time so as to cooperate with the driving stage to maintain the drive current itself at a predetermined reference value.
In particular, the driving stage typically comprises a bipolar junction transistor, having the collector connected to one of the two terminals of the lighting branch, the emitter connected to a ground line through a resistor and the base connected to an output terminal of the control stage to receive the control signal from the latter.
Such electronic control devices have the drawback that the total electric power drawn by the transistor and by the resistor of the driving stage is relatively high if compared with the electric power drawn by the lighting branch, and is prevalently transformed into heat by Joule effect, thus causing the overheating of the driving stage.
For this purpose, the driving stage is provided with a heat sink device, which consists of a heat sink area made of copper, which occupies a portion of the electronic board which houses the printed circuit and the electronic components of the driving stage itself, in turn. The presence of the heat sink area causes a considerable increase of the total dimensions of the electronic board, which in some cases becomes too big to be installed in the rear hull of the light.
In order to overcome the aforesaid technical problem, it has been suggested to use two transistors and two resistors in the driving stage instead of a single transistor and a single resistor, to position them on two different electronic boards and to equip the electronic boards with two respective heat sink areas of smaller size than the heat sink area used in the single electronic board solution. Each of the two heat sink areas is indeed dimensioned to dissipate the heat generated by the respective transistor and resistor. In the case in point, the two bipolar junction transistors have respective collectors connected to a common terminal of the lighting branch, the emitters connected to a ground line by way of respective so-called “decay resistors”, and the bases both connected to an output terminal of the control stage to receive the common control signal from the latter. It is worth noting that the fractioning of the heat sink area of the single electronic board in the two distinct heat sink areas positioned on the two independent electronic boards advantageously allows to reduce the dimensions of each electronic board, which may consequently be installed within the automotive light in a position facing the other board, even when the available space in the light is particularly small.
Although, on one hand, the solution with double electronic board and double transistor/resistor allows to reduce the size of the heat sink areas and the dimensions of the two electronic boards, on the other hand it has a series of technical problems which have not yet been solved.
Firstly, the impossibility to have two identical transistors, i.e. having the same features in terms of shape, amplification, according to temperature variations etc. makes it complex to determine the electric power actually drawn by each transistor/resistor present in each electronic board in accurate manner. Tests carried out by the Applicant have indeed demonstrated that the relative error associated with the estimated splitting of the electric powers drawn by the two transistors/resistors present in the two electronic boards is higher than 20% of the total power drawn by both boards. Consequently, such an error margin makes it difficult to dimension the two heat sink areas in the two boards, which consequently may be inadequate unless they are appropriately overdimensioned.
Furthermore, the driving stage requires an availability of a particularly high supply voltage, which in some cases may be higher than that actually available at the terminals of each driving branch, constituted by the transistor and by the decay resistor, in use. It is indeed known that the electronic control device and the lighting branches are typically supplied by a main supply voltage, the value of which is established by the automotive manufacturers and generally varies between 0 and 18 volt. One of the objectives of automotive manufacturers is to be able to reduce the main supply voltage of the electronic control device so as to obtain the lighting of the LEDs with a low voltage. For example, the lighting voltage of the lighting branches each provided with two LEDs connected in series to each other, typically equal to 7 Volt, of which one part is needed to supply the two LEDs and is equal to approximately 5.3 Volt, while the remaining part is needed to compensate for the voltage drop of the remaining active and passive electronic components of the control device, and is typically equal to 1.6-1.7 V. Tests carried out by the Applicant have however demonstrated that in an electronic control device provided with the driving stage with double board, double transistor and double decay resistor configuration, the voltage required to supply the active and passive electronic components is at least 2 Volt, i.e. higher than the available 1.7 Volt.
A further technical problem of the electronic control device in which the driving stage is housed on the double electronic board is based on the fact that the common control signal having a relatively low intensity is supplied by the first board to the second electronic board by way of a specific external connection, and consequently it is subject to electromagnetic interference, which may vary the intensity causing an alteration of the operation of the transistor present in the second board itself.